How to offset the start of rigid body physics to a specific frame
To offset when a rigid body simulation starts, I would suggest that you use keyframing to your advantage. You can keyframe any option in blender, including the "Animate" and "Dynamic" check boxes in the rigid body options. Mouse over the "Animate" or "Dynamic" checkbox and press "i" like any other add keyframe.


You can then make it so that only when the object is hit, or at a certain time, will the object begin to fall. Likewise, you can also use this so that your fractured objects do not fall apart until they are hit or about to be hit. Hope this helps!

P.S.You can find a full tutorial detailing this sort of animation/simulation here. This is a three part tutorial (with a prequel teaching how to model the mug) made by Blender Cookie.